Control Strategies for Whitefly as a Vector Elysia Slakie, Caitlin McKee, Angela Gaffney, for Cassava Viral Diseases C. Leigh Anderson & Mary Kay Gugerty EPAR Brief No. 233 Prepared for the Agricultural Policy Team of the Bill & Melinda Gates Foundation Professor Leigh Anderson, Principal Investigator Associate Professor Mary Kay Gugerty, Principal Investigator May 21, 2013 EPAR’s innovative student-faculty team model is the first University of Washington partnership to provide rigorous, applied research and analysis to the Bill and Melinda Gates Foundation. Established in 2008, the EPAR model has since been emulated by other UW Schools and programs to further support the foundation and enhance student learning. NOTE: The findings and conclusions contained within this material are those of the authors and do not necessarily reflect positions or policies of the Bill & Melinda Gates Foundation. EVANS SCHOOL POLICY ANALYSIS AND RESEARCH (EPAR) | 1 Note: This draft is currently under revision to incorporate scientist comments. Section I: Overview & Summary Cassava is an important crop, particularly in food-insecure regions. It ranks 10 th by quantity (19 th most important by value) of crops worldwide, with 252,203,769 tonnes produced in 2011 (FAOStat). Cassava yields are compromised by pests such as whiteflies, mites, and weevils, which cause significant crop losses through the spread of viral disease and direct damage to plants. Whiteflies are vectors for viral diseases such as cassava mosaic disease (CMD) and cassava brown streak disease (CBSD), which can reduce yields by up to 40% (Legg & Fauquet, 2004). The flies can also cause direct damage by feeding on the crop and causing root yield reductions. Direct damage is more common on disease-resistant cassava varieties than on local cultivars due to higher whitefly populations on disease-resistant plants, illustrating the uniquely problematic vector- host relationship between cassava and this pest. This report provides background on whitefly damage to cassava as well as summaries of the evidence on the efficacy of four control strategies: • Breeding for host plant resistance • Intercropping and other planting strategies • Insecticides and Insecticide Resistance Management (IRM) • Biological control through parasitoids, fungus, and predators Though we group information by strategy, most researchers report that controlling whiteflies and the viruses for which it is a vector requires a multifaceted approach. As Thresh & Cooter (2005) suggest, the measures to control CMD should be “simple, inexpensive, and within the limited capacity of the farmers’ concerns.” The four whitefly control strategies are summarized below and discussed in further detail in Section IV. Summary of Control Strategies Breeding for host plant resistance: While breeding resistance has received attention among researchers, whiteflies species adapt quickly, which renders plant resistance temporary. Researchers have identified genotypes that show resistance to whiteflies. These genotypes could provide the parental genotypes required for future breeding programs (Omongo, 2012). However, whitefly-resistant varieties are likely to be CMD-susceptible, so when whitefly-resistant varieties are identified, further breeding may be necessary to develop varieties that are resistant to both whiteflies and viruses (Thresh & Cooper, 2005). Centro Internacional de Agricultura Tropical (CIAT) is currently working on identifying genes that confer resistance to whitefly. Intercropping and other planting strategies: Intercropping is an environmentally benign method to control whiteflies. It is associated with lower whitefly populations, CMD incidence, and severity (Night, 2011). This planting strategy has the potential to decrease the need for insecticide use
Transcript
Control Strategies for Whitefly as a Vector Elysia Slakie, Caitlin McKee, Angela Gaffney, for Cassava Viral Diseases C. Leigh Anderson & Mary Kay Gugerty EPAR Brief No. 233
Prepared for the Agricultural Policy Team of the Bill & Melinda Gates Foundation
Professor Leigh Anderson, Principal Investigator Associate Professor Mary Kay Gugerty, Principal Investigator May 21, 2013
EPAR’s innovative student-faculty team model is the first University of Washington partnership to provide rigorous, applied research and analysis to the Bill and Melinda Gates Foundation. Established in 2008, the EPAR model has since been emulated by other UW Schools and programs to further support the foundation and enhance student learning. NOTE: The findings and conclusions contained within this material are those of the authors and do not necessarily reflect positions or policies of the Bill & Melinda Gates Foundation. EVANS SCHOOL POLICY ANALYSI S AND RESEARCH (EPAR) | 1
Note: This draft is currently under revision to incorporate scientist comments.
Section I: Overview & Summary
Cassava is an important crop, particularly in food-insecure regions. It ranks 10th by quantity (19th most important by value) of crops worldwide, with 252,203,769 tonnes produced in 2011 (FAOStat). Cassava yields are compromised by pests such as whiteflies, mites, and weevils, which cause significant crop losses through the spread of viral disease and direct damage to plants. Whiteflies are vectors for viral diseases such as cassava mosaic disease (CMD) and cassava brown streak disease (CBSD), which can reduce yields by up to 40% (Legg & Fauquet, 2004). The flies can also cause direct damage by feeding on the crop and causing root yield reductions. Direct damage is more common on disease-resistant cassava varieties than on local cultivars due to higher whitefly populations on disease-resistant plants, illustrating the uniquely problematic vector-host relationship between cassava and this pest.
This report provides background on whitefly damage to cassava as well as summaries of the evidence on the efficacy of four control strategies:
• Breeding for host plant resistance
• Intercropping and other planting strategies
• Insecticides and Insecticide Resistance Management (IRM)
• Biological control through parasitoids, fungus, and predators
Though we group information by strategy, most researchers report that controlling whiteflies and the viruses for which it is a vector requires a multifaceted approach. As Thresh & Cooter (2005) suggest, the measures to control CMD should be “simple, inexpensive, and within the limited capacity of the farmers’ concerns.” The four whitefly control strategies are summarized below and discussed in further detail in Section IV. Summary of Control Strategies Breeding for host plant resistance: While breeding resistance has received attention among researchers, whiteflies species adapt quickly, which renders plant resistance temporary. Researchers have identified genotypes that show resistance to whiteflies. These genotypes could provide the parental genotypes required for future breeding programs (Omongo, 2012). However, whitefly-resistant varieties are likely to be CMD-susceptible, so when whitefly-resistant varieties are identified, further breeding may be necessary to develop varieties that are resistant to both whiteflies and viruses (Thresh & Cooper, 2005). Centro Internacional de Agricultura Tropical (CIAT) is currently working on identifying genes that confer resistance to whitefly. Intercropping and other planting strategies: Intercropping is an environmentally benign method to control whiteflies. It is associated with lower whitefly populations, CMD incidence, and severity (Night, 2011). This planting strategy has the potential to decrease the need for insecticide use
EVANS SCHOOL POLICY ANALYSI S AND RESEARCH (EPAR) |
2
and is already commonly practiced by many smallholder farmers. Evidence shows that higher density intercropping is most effective in controlling whitefly populations (Fargette & Fauquet, 1990). However, the precise planting densities at research stations may differ from what smallholder farmers practice, so results may vary when replicated in the field. Insecticides and Insecticide Resistance Management (IRM): Widespread insecticide use has historically been viewed as an ineffective and environmentally damaging strategy to control whitefly populations due to heightened whitefly resistance (Horowitz et al., 2011). However, more recent research shows that new, targeted insect growth regulators (IRGs), in combination with insecticide resistance management (IRM) efforts such as refuge strategies, can increase the effectiveness of these control strategies and delay the resistance of susceptible pests (Carriere et al., 2012). Much of the insecticide and insecticide resistance research has focused on commercial crops such as cotton and ornamental plants rather than cassava.
Biological control through parasitoids, fungus, and predators: While natural enemies alone do not typically solve whitefly problems (Horowitz et al., 2011), introducing enemies and biological control as part of an integrated pest management system (IPM) may prove more effective. Biological control mechanisms shown to be successful against whiteflies include parasitoids, predators, and fungal control. Exotic parasitoids or predators have been used successfully against whitefly in other crops (Gerling et al., 2001) and may be effective for cassava in some cases after careful suitability studies (Aiisime et al., 2007). Other control methods (breeding for resistance, insecticide) can negatively impact natural enemies, making whitefly control more challenging over the long-term.
This report provides several appendices for reference. Appendix 1 lists confirmed and proposed viral species spread by whitefly; Appendix 2 lists natural enemies of whitefly on cassava, which may be important for biological control; and Appendix 3 lists notable researchers and their affiliated institutions. To provide context for current research and interventions, Appendix 4 provides a summary table of empirical studies by control mechanism, and Appendix 5 provides a summary table of intervention programs with a whitefly component. Section II: Methodology and Research Context
We searched for peer-reviewed journal articles and gray literature using Google, Google Scholar, and the University of Washington Library system using combinations of the words: whitefly, Bemisia tabaci, cassava, disease control, strategies, pest management, intercrop, breeding, resistance, and insecticide, among others. We generally limited literature searches to 1990 or later. While we primarily include literature specifically related to cassava, we also include results of research and interventions on other host crops. We have mostly excluded research on whitefly control studies in greenhouses.
Much of the literature on whitefly control mechanisms focuses on commercial crops, and the cassava-specific literature focuses primarily on whitefly as a vector for viral diseases. However, recent publications acknowledge the increasing threat of direct damage from whiteflies and increased risks of new virus types in areas with superabundant1 whitefly populations.
Research on whitefly control strategies has been concentrated in two research centers: Centro Internacional de Agricultura Tropical (CIAT) in Colombia has focused on A. socialis species of whitefly common in the Americas and International Institute of Tropical Agriculture (IITA) in Nigeria has done research on B. tabaci common in Africa.
Section III: Background on Whitefly and Cassava Viral Diseases Whiteflies Entomologists have identified approximately 1,500 species of whitefly; Bemisia tabaci (B. tabaci) is the most common species to which crop losses are attributed in tropical regions (Legg et al., 2003). Whiteflies are difficult to control because they breed multiple times in a year (multivoltine), evolve rapidly, and have a broad range of plant hosts (extreme polyphagy) (Asiimwe et al., 2007).
1 Superabundant whitefly populations are typically 100-times greater than populations outside the CMD pandemic zone (Legg, 2009).
EVANS SCHOOL POLICY ANALYSI S AND RESEARCH (EPAR) |
3
In the Americas, several species of whitefly (Aleurotrachelus socialis and Trialeurodes variabilis) are considered to be among the major cassava pests, while in Africa and South Asia Bemisia tabaci (B.tabaci) is the most prominently cited species (Bellotti 2012). Aleurodicus disperses may be causing yield loss in Asia, though research on this species and the damage it causes is limited (Bellotti et al., 2012b). Table 1 from Bellotti et al., (2012b) shows the distribution of major whitefly species causing damage to cassava. Table 1: Major Whitefly Species
Major Species Americas Africa Asia
Aleurotrachelus socialis x
Aleurothrixus aepim x
Aleurodicus dispersus x x x
Aleurodicus flavus x
Aleuronudus sp. x
Bemisia afer x x
Bemisia tuberculata x
Bemisia tabaci x x x
Paraleyrodes sp. x
Tetraleurodes sp. x
Trialeurodes vaporariorum x
Trialeurodes variabilis x
Source: Bellotti et al., 2012b Bemisia tabaci (B. tabaci) B. tabaci is the most common whitefly species to which cassava crop damage and disease are attributed in tropical regions. Over 900 host plant species are associated with B. tabaci, and they transmit 111 virus species (Global Invasive Species Database). According to Carabalí (2010), scientists generally agree that B. tabaci is a “complex of morphologically indistinguishable populations with different biological biotypes.” Both B and Q Biotypes of B. tabaci have been identified, and while the B-Biotype is particularly prevalent on crops in Africa, the Q-Biotype has been more recently identified as a pest to cotton and ornamental plants in the United States (Patil & Fauquet, 2009). Some researchers consider the B type a separate species (B. argentifoloo) (Bellotti et al., 2012b). Insecticide resistance, damage to plants, and virus transmission ability varies between biotypes. The inability of the B-Biotype to colonize cassava in the Americas is postulated as the reason CMD has not spread to this region (Carabali, et al., 2005). A study in Africa found that different types of B. tabaci found on cassava in different geographic areas breed readily, but sweet potato whiteflies do not breed with cassava whiteflies (Maruthi et al., 2001 in Legg et al., 2006). Whitefly-caused damage Whiteflies damage cassava directly and are a vector for disease. Whiteflies are vectors for five broad categories of viruses: Begomovirus, Ipomovirus, Crinivirus, Carlavirus, and Torradovirus but most of the virus species are in the Begomovirus genus (Geminiviridae family, also referred to as geminiviruses (Legg, 2009; Navas-Castillo et al., 2011). Appendix 1 provides a list of all accepted and proposed virus species transmitted by the flies as of 2011. Cassava mosaic disease (CMD) (caused by Begomovirus) and cassava brown streak disease (CMSD) (caused by Ipovirus) cause the most damage in cassava (Legg, 2009). The flies can also cause direct damage by feeding on the crop and causing root yield reductions. Whiteflies also cause disease and damage on tomatoes, pepper, cucurbits, beans, sweet potatoes and other crop species and are a problem for both commercial and small-scale farmers (Morales, 2006). Cassava Mosaic Disease: CMD causes a yellowing and distortion of leaves and results in stunted growth. Losses from CMD spread by whiteflies and infected cuttings resulted in production losses up to 47% in East and Central Africa from the early 1990’s to 2006 (IITA, n.d.). Despite widespread cassava cultivation in Latin America and several Southeast Asian countries, CMD has only been
EVANS SCHOOL POLICY ANALYSI S AND RESEARCH (EPAR) |
4
reported in the African continent and Indian subcontinent (Patil & Fauquet, 2009). CMD is not caused by a single virus, but a collection of several related begomoviruses (Navas-Castillo et al., 2011). Among cassava-infecting viruses, these include:
o African cassava mosaic virus o East African cassava mosaic virus o East African cassava mosaic Cameroon virus o East African cassava mosaic Kenya virus o East African cassava mosaic Malawi virus o East African cassava mosaic Zanzibar virus o South African cassava mosaic virus o Indian cassava mosaic virus o Sri Lankan cassava mosaic virus
Though CMD-resistant cassava varieties have been successfully introduced in many of the affected areas, the new CMD- resistant varieties appear to be particularly susceptible to whitefly infestation. Cassava Brown Streak Disease: Though cassava brown streak disease (CBSD) is less widespread and has received less attention than CMD, it causes major losses due to root necrosis and is now the most significant threat to cassava in East Africa (Hillocks, 2003; Mbanzibwa et al., 2011). Like CMD, CBSD is not caused by a single virus. Two species in the Ipomovirus genus, cassava brown streak virus, and the more recently described Ugandan cassava brown streak virus, are associated with CBSD (Mbanzibwa et al., 2011). Maruthi et al. (2005) established whiteflies are a vector for CBSD, but transmission is low and infected plant cuttings are likely a more significant cause of infection. Whiteflies are a vector only over distances of less than 50m, and the virus is transmitted during a short time period of infectivity (RCI- Mid-Term Review, 2012). However new infections of CBSD are associated with peaks in whitefly populations, and Maruthi et al. (2005) suggest experimental conditions may minimize transmission of CBSD by whiteflies relative to natural conditions. Mbanzibwa et al. (2011) report that recombination is a mechanism for both virus species and evolution could be accelerated as the viruses spread to new geographic areas. Emerging Viruses According to Navas-Castillo et al. (2011), in addition to the major cassava-affecting viral diseases, “it is not very risky to assert that a new virus will emerge when given the opportunity.” The underlying causes of the emergence of new whitefly-transmitted viral plant diseases are multifactorial and result from a combination of changes in virus, vector, host, and environment. Factors that may contribute to emergence include recombination and synergism between virus species, new vector biotypes, genome integration of the virus and host adaptation (Fargette et al., 2006 ). Other factors that increase whitefly-transmitted diseases emerging include host range expansion, long-distance traffic of plant materials or insects, agro-ecological changes, agricultural intensification, and climate change.
Appendix 1 provides a comprehensive list of whitefly-hosted virus species, including the 111 that B. tabaci specifically host. The vast majority of viruses are begomoviruses (there are nine that cause cassava mosiac disease), though two ipomoviruses are important to cassava as well (these cause cassava brownstreak disease). In recent years, the numbers of new begomovirus species as well as new hosts and geographical distributions for known begomoviruses have increased substantially in Latin America, particularly in Brazil and in the Caribbean. The two more recent cases of emerging recombinant begomoviruses that have been well documented include tomato yellow leaf curl disease (TYLCD) in Southern Spain and a Ugandan strain of East African cassava mosaic virus. Other important emerging diseases affecting vegetable crops include diseases caused by bipartite begomoviruses in Latin America, ipomovirus diseases of cucurbits, tomato chlorosis caused by criniviruses, and the torrado-like diseases of tomato.
While Fauquet et al. (1998) argue that evolutionary changes in geminiviruses such as the recombination between ACMV and EACMV to produce UgV/EACMV-UG (the virus responsible for the CMD epidemic in Uganda from 1989-1991) may be relatively common, Legg & Thresh (2000) assert that the frequency with which these recombinations between two distinct cassava mosaic geminiviruses become epidemiologically significant is relatively low, which they say is encouraging for CMD management in the 21st century.
EVANS SCHOOL POLICY ANALYSI S AND RESEARCH (EPAR) |
5
Controlling emerging whitefly-transmitted diseases is expected to require regional solutions (Navas-Castillo et al., 2011). In the case of a tomato mottle virus emergence in Florida, Navas-Castillo et al. describe the state’s establishment of a synchronized tomato-free period and facilitated growers’ use of transplants imported from distant, infection-free areas. Direct damage from whiteflies Direct crop damage occurs when whiteflies feed on plant phloem, removing plant sap and reducing overall plant vitality. Whiteflies also excrete honeydew, which promotes sooty mold on leaves that interferes with photosynthesis and damages harvest quality (Navas-Castillo et al., 2011). A. socialis, A. aepim, and T. variabilis cause yield loss through direct feeding in the Americas. B. tabaci on CMD-resistant cultivars: Until recently, whiteflies in Africa primarily harmed cassava as a viral vector, but beginning in the 1990’s, superabundant whiteflies have also caused damage to cassava through direct feeding on plant leaves. Superabundant whitefly populations may be caused by a particular B. tabaci biotype or as a result of interaction between B. tababci and CMD-infected cassava. However, for unknown reasons, the superabundant populations are found on CMD-resistant varieties (Legg, 2009). Local cultivars in East Africa are highly susceptible to CMD but relatively unaffected by direct whitefly feeding, while improved, resistant varieties have yield losses from direct damage ranging from 12.5-44.6%, in part because CMD-resistant plants host larger populations of B. tabaci (Stansly & McKenzie, 2008). High populations of B. tabaci in Uganda are due in part to adoption of CMD-resistant cultivars (Omongo et al., 2012). The large populations of whiteflies feed on both the CMD resistant varieties and susceptible varieties planted nearby. This reduces the effectiveness of interventions aimed at breeding or dissemination of clean, virus-resistant cuttings and increases the risk of new virulent virus strains (Asiimwe et al., 2007). The cassava geminiviruses can recombine; recombination is more likely when whiteflies are superabundant (Legg et al., 2003).
Farmer awareness of whitefly as a disease vector Many farmers have low awareness of whiteflies as vectors of CMD, as shown by surveys conducted with the ESCaPP program in the mid-90’s in various countries in SSA. For example in Benin, whiteflies were present at all 60 research sites with an average of 0.5-3.2 flies per plant. However, the survey showed that 60% of farmers did not know that whiteflies caused damage to cassava. Those farmers did not have a specific word for whiteflies in the local language. They identified whiteflies with the general word “insect”, implying a need to increase biological awareness. This may suggest that farmers’ perception of the incidence of CMD was much lower than field data show (Anderson, 2005). Among farmers in various districts in western Kenya, several other species ranked as more important cassava pests, including mealy bugs, moles, green cassava mites, scales, and termites (IITA, 1999). We did not find more recent information on farmer perceptions of whitefly as a disease vector. Section IV: Whitefly Control Strategies in the Literature The following section provides an overview of recent, available literature on each of the whitefly intervention strategies, as summarized in Section I. Breeding for Host Plant Resistance (HPR) While breeding resistance has received attention among researchers, whitefly species adapt quickly, which renders plant resistance temporary. Whitefly“resistant” cassava varieties means that they can depress whitefly populations by reducing whitefly oviposition, lengthening the development period, and causing high nymphal mortality. If whitefly populations are depressed, then other methods of biological control (discussed below) can be more effective (Bellotti, 2012a). Researchers have mapped the cassava genome (Prochnik et al., 2012) and identified genes conferring resistance to CMD (Akano, 2001; Okogbenin et al, 2012). Transgenic varieties have been developed with resistance to CMD and CBSD (Zhang, 2005; Vanderschuren, 2012; Patil, 2011). Whitefly resistant varieties are likely to be CMD susceptible, so when whitefly resistant varieties are identified, further breeding will likely be needed to develop varieties that are resistant to both
EVANS SCHOOL POLICY ANALYSI S AND RESEARCH (EPAR) |
6
whitefly and viruses (Thresh & Cooper, 2005). Since whiteflies also cause direct damage to cassava plants, Legg et al. (2006) suggest researchers should also consider selectively breeding to reduce direct damage. Researchers at CIAT have used Simple Sequences Repeat (SSR) to identify markers associated with genes causing whitefly (A. socialis) resistance in MEcu 72. Preliminary framework maps were presented by Bellotti et al (2003). Bellotti et al., (2012b) indicates the whitefly resistance gene tagging project is ongoing. CIAT scientists report that initial results suggest whitefly resistance may involve multiple genetic regions (Bohorquez et al, 2011). Researchers have identified genotypes that show resistance to whiteflies, and transgenic breeding can be precise in introducing whitefly resistant genes (Legg et al., 2006).These genotypes could provide the parental genotypes required for future breeding programs (Omongo, 2012). CIAT research has demonstrated that cassava genotype MEcu 72 shows high levels of A. socialis resistance (with 72.5% nymph morality) and MEcu64, MPer 334, MP415, and MP273 express moderate-to-high resistance (Bellotti, 2012a). MEcu 72 was introduced in Uganda in 2005 (Bellotti et al., 2012b). Ugandan landraces have also been found to be resistant to whiteflies and include Ofumba Chai, Nabwire 1, and Mercury (Ugandan landraces) (Omongo, 2012a). The promising A. socialis resistant variety of cassava called Nataima-31 was bred in Colombia using MEcu 72. It was selected from various progeny because of its high yield and good cooking quality attributes. Without any insecticide application, it has attained yields of 33 t/ha which exceeds regional farmer’s output by 34%. It is now being grown commercially in Colombia, Ecuador and Brazil (Bellotti, 2012a; Arias, 2004; Vargas, 2002). Results from a CIAT study show the potential of gene introgression for pest resistance. Akinbo (2012) confirmed introgression of resistance to A. socialis after evaluating 227 genotypes and finding 17.8% promising for future breeding because they had low whitefly damage ratings, indicating high resistance. CIAT is currently using advanced back cross (ABC) Quantitative Trait Locus (QTL) to introgress genes for whitefly resistance (CIAT, 2012). Using molecular markers ABC-QTL mapping can more efficiently use introgression of useful genes from wild relatives to cultivated varieties (Bellotti, 2012b). Phylogenetically related hosts such as Jatropha gossypiifolia can act as intermediate hosts in which whiteflies increase their biotic potential and ability to adapt to cassava. CIAT tested the ability of B. tabaci type B to colonize different varieties of cassava (one commercial M. esculenta and two wild M. flabellifolia, M. carthaginensis) after coming from host sequences of other species based on survival and oviposition rates. Carabalí (2010) found that 60% of whiteflies could reproduce on the wild cassava species, 55% on J. gossypiifolia, and 27.5% on the commercial variety. Phylogenetic analysis shows that a new strain of Indian cassava mosaic virus (transmitted to jatropha through whitefly) causes jatropha curcas mosaic disease (Gao, 2010). Proposed plans for jatropha as a major biofuel crop in the Americas could increase the possibility that B. tabaci could adapt to cassava as a host in the Americas and raises the risk of B. tabaci-spread viruses in the region (Bellotti et al., 2012b). Intercropping and Planting Strategies
Intercropping is an environmentally benign method to control whiteflies and is associated with lower whitefly populations, CMD incidence, and disease severity (Night 2011). This planting strategy has the potential to decrease the need for insecticide use and is already commonly practiced by many smallholder farmers. Evidence shows that higher density intercropping is most effective in controlling whitefly populations (Fargette & Fauquet, 1990). High density planting on cassava plots is more important than height barriers of intercropped or edge crops such as maize to control whitefly populations (Fargette & Fauquet, 1988). However, the precise planting densities at research stations may differ from what smallholder farmers practice, so results may vary when replicated on farms.
Different crops intercropped with cassava produce varying results in reducing whitefly populations. Cowpeas were shown to be more effective than maize in reducing whitefly egg densities on cassava leaves (Gold, 1990). The type of intercropping also has an impact on whether intercropped cassava has greater or lesser yields when compared to monocropped plots. Intercropping with maize has been shown to reduce cassava yield (Olasantan et al., 1996) while intercropping with legumes has been shown to increase cassava yield (Islami et al., 2011; Njoku & Muoneke, 2008). Thus, cowpeas generally are beneficial for reducing whitefly populations and increasing cassava yields while maize may not have the same overall
EVANS SCHOOL POLICY ANALYSI S AND RESEARCH (EPAR) |
7
positive benefits. Some crops should not be planted near cassava. Bellotti (2012b) recommends not planting jatropha in proximity to cassava due to whiteflies’ ability to adapt to previously resistant cassava via other plant hosts.
Plant architecture is also an important factor influencing whitefly populations. Intercropping with certain crops can reduce cassava plant and leaf size since whitefly egg densities can be less on smaller leaves (Gold, 1990). However, different studies find varying impacts of intercropping on cassava leaf size. Gold (1990) found that intercropping cassava can reduce leaf size due to competition from maize and cowpeas, and Olasantan (1996) found the Leaf Area Index (LAI)2 to be lower in cassava intercropped with maize. However, Njoku & Muoneke (2008) found that the LAI was higher or similar in plots intercropped with cowpeas. While intercropping can influence the LAI (Njoku, 2008), it is also heavily influenced by genotype, plant age, and environment. (IITA, n.d.).
Intercropping may be effective at controlling whiteflies because it changes the microclimate or field ecosystem, altering their movement and behavior (Fondong, 2002) or because competition for nutrients in an intercropped setting alters cassava plants (Olasantan, 1996; Gold, 1990). Impacts of intercropping to reduce whitefly populations on cassava leaves varies at different times of the growing cycle with the greatest reductions occurring post-harvest of the intercropped crop (Fondong, 2002; Gold, 1990).
Most studies rely on counting eggs, nymphs or adult whiteflies on cassava plants to determine if intercropping is an effective method for controlling populations. However counting methods are not consistent. Since whiteflies have a restless behavior and the numbers seen on leaves depend on the time of day and weather conditions, the measured effectiveness of intercropping on those populations will vary depending on how they are counted (Sseruwago et al., 2004; Abisgold & Fishpool, 1990). While intercropping can reduce whitefly populations on cassava plants, it does not eliminate the flies completely. The linkage between whiteflies and CMD remains strong: among intercropped plots, those with higher populations of whiteflies also have greater incidence of CMD (Fondong, 2002).
Insecticides and Insecticide Resistance Management (IRM)
Widespread insecticide use has historically been viewed as an ineffective and environmentally damaging strategy to control whitefly populations (Horowitz et al., 2011). Insecticide application on cassava is particularly challenging due to the location of flies (under leaves), their highly polyphagous nature, and their easy dispersion by the wind (Horowitz et al.; Navas-Castillo et al., 2011). While applying insecticide can reduce whitefly populations, the CRS Great Lakes Cassava Initiative report found that insecticides did not stop the spread of CBSV, and plots treated with insecticide were more susceptible to CBSD (Catholic Relief Services, n.d.). According to Castle et al. (2010), over-reaching insecticide use has resulted in heightened resistance among whiteflies, “tipping the balance between a manageable infestation and uncontrolled outbreak.” The majority of the literature agrees with the limited efficacy of insecticide-based control strategies due to environmental concerns and resistance (Horowitz et al., 2011). In addition to environmental and health consequences, Thresh & Cooter (2005) advocate against insecticide use to avoid harming natural predators. Evidence from cotton (Eveleens, 1983; Dittrich et al., 1985) suggests that insecticides are more effective against natural enemies than against whiteflies, which can lead to population resurgence after insecticide use. Bellotti further agrees that farmers using insecticides to control whiteflies will also reduce the effectiveness of biological control (Bellotti in Anderson, 2005). Despite the general consensus that widespread insecticide use is an ineffective control strategy, more recent research indicates that newer, more targeted insecticides and insect growth regulators (IRGs) are preferable because of their ability to target specific pests, their effectiveness at low application rates, and their non-persistent characteristics in the environment. Further, their selectivity renders many of them suitable for IPM programs (Casida & Quistad, 1998).
2 Leaf Area Index (LAI) is the ratio of leaf area per unit of ground area. Higher LAI means greater “leafiness” of the plant and cassava genotypes with high LAI have higher root yields (IITA n.d.).
EVANS SCHOOL POLICY ANALYSI S AND RESEARCH (EPAR) |
8
Newer, more selective IGRs have not been mentioned for use on cassava specifically (Horowitz et al., 2011). Producers in the U.S. have had the greatest success with novel insecticide chemistries such as Nicotinoids, Imadacloprid soil treatments, second-generation nicotinoids, and non-neurotoxic IGRs such as buprofezin and pyriproxyfen (Palumbo, et al., 2001). Insecticide resistance management strategies based on the structured and restricted use of non-neurotoxic IGRs, coupled with the use of cultural and biological pest management tactics, are presently held to provide the best model for combating insecticide resistance in B. tabaci (Ellsworth et al., 2001).
In combination with IRGs, refuge strategies can be effective in preventing resistance to pyriproxyfen. Spring melon, alfalfa, and cotton not treated with insecticides provide refuge for B. tabaci and promote their survival. Results may be useful to predict the spatial determination of a refuge strategy. Cotton refuges delayed pest resistance while treated cotton fields accelerated it (Carriere et al., 2012). Insecticide resistance management (IRM) strategies are important in addition to insecticides to “incorporate newer chemistries into viable control programs that emphasize conservation of natural enemies and active ingredients” (Castle et al., 2010).
Biological Control While natural enemies alone do not typically solve B. tabaci problems (Horowitz et al., 2011), introducing enemies and biological control as part of an integrated pest management system (IPM) may prove more effective. Biological control methods can be combined and used with other pest management techniques such as intercropping (Legg et al., 2003). Other whitefly control methods (breeding for resistance, insecticide) can negatively affect natural enemies, making whitefly control more challenging over the long-term. Various biological control methods may also be incompatible. For example, some fungi that suppress whitefly also affect whitefly predators and parasitoids. Biological control mechanisms shown to be successful against whitefly include parasitoids, predators, and fungal control. Exotic parasitoids or predators have been used successfully in other crops (Gerling et al, 2001) and may be effective for cassava in some cases after careful suitability studies (Aiisime et al, 2007). Biological control was initially dismissed as a control mechanism on cassava because until the 1990’s whitefly was not recognized as causing substantial direct damage. Interest was renewed when direct damage was noted in Uganda and elsewhere (Thresh & Cooter, 2005). Introducing exotic enemies poses risks, but may be more effective at controlling than local natural enemies. Natural enemies can be introduced, conserved, or augmented (Bellotti et al., 2012a). Conservation or augmentation of local natural enemies may be an effective strategy, particularly in areas where insecticide use has changed natural balances between pests and enemies (Legg et al., 2003). Parasitoids/predators: Research is ongoing to identify natural enemies of whitefly and design interventions to use them for controlling whiteflies (Legg et al., 2006). Gerling et al. (2001) and Arno et al. (2010) identify 38 spider species and 123 insect species that are predators of B. tabaci (Horowitx et al., 2011). Predators are used primarily in greenhouse conditions and several are available commercially (Horowitz et al., 2011). However, predators may be specific to the host plant (Horowitz et al., 2011), so results from studies of other plants may not be applicable to cassava. Bellotti et al. (2012a) identifies Delphastus pusillus, and Condylostylus as whitefly predators on cassava. Aiisime et al. (2007) recommends conserving and/or enhancing parasitism to control whiteflies by developing cassava varieties that resist B. tabaci, but encourage survival of parasitoid species. They also recommend introducing exotic parasitoids (after careful suitability studies). While parasitoids have been used most commonly in greenhouses, exotic parasitoids have also been introduced to control whiteflies on outdoor crops and nurseries (Gerling et al., 2001), including Eretmoccerus in Australia (De Barro & Coombs, 2008) and in Arizona, USA (Gould et al., 2008). Bellotti et al. (2012), identifies six parasitoids of B. tabaci on cassava: Ecarsia Sophia, E. lutea, E. Formosa, E. mineaoi, Encarsia sp., and Eretmocerus mundus. Appendix 2 provides Bellotti’s table of enemies for all species of whitefly that feed on cassava. Introduction of an exotic parasitoid to Africa successfully and economically controlled cassava mealybug and green mite, suggesting biological control of whiteflies could be feasible (Bellotti et al., 2012a; Bellotti et al., 2012b). Fungal products/control:
EVANS SCHOOL POLICY ANALYSI S AND RESEARCH (EPAR) |
9
Products based on fungi, (Verticillium lecanii, Paecilomyces fumosoroseus and Beauveria bassiana) have the capacity to suppress whitefly (Faria & Wraight, 2001). Horowitz et al. (2011) also notes Aschersonia and Metarhisum as infectious to whitefly. Beauveria bassiana is sold as Eco-Bb by Plant Health Products3 for the control of whiteflies in South Africa and Zambia on beans, tomatoes, cucumbers, and eggplant. Constraints to effective use of fungal whitefly control include “slow action, poor adulticidal activity, potentially negative interactions with commonly used fungicides, relatively high cost, limited shelf life, and dependence on favorable environmental conditions” (Faria & Wraight, 2001). Bellotti et al. (2012b) notes these products appear to only be successful when applied when whitefly populations are low. Fungal pathogens can be delivered by spraying the underside of the crop leaves, but Faria & Wraight (2001), note in a paper geared towards commercial agriculture that cost is prohibitive. This suggests fungal products are unlikely to be economically feasible for small-scale cassava farmers. While fungi with potential for whitefly control do not pose risks for vertebrates, some types infect whitefly predators and parasites, potentially limiting these other mechanisms of whitefly control (Faria & Wraight, 2001).
Please direct comments or questions about this research to Leigh Anderson and Mary Kay Gugerty, at [email protected].
EVANS SCHOOL POLICY ANALYSI S AND RESEARCH (EPAR) |
10
References
Abisgold, J. D., & Fishpool, L. D. C. (1990). A method for estimating population sizes of whitefly nymphs (Bemisia tabaci
Genn.) on cassava. International Journal of Pest Management, 36(3), 287-292.
Akano, A., Dixon, A., Mba, C., Barrera, E., & Fregene, M. (2002). Genetic mapping of a dominant gene conferring resistance to cassava mosaic disease. Theoretical and Applied Genetics, 105(4), 521-525.
Akinbo, O., Labuschagne, M., & Fregene, M. (2012). Introgression of whitefly (Aleurotrachelus socialis) resistance gene from F1 inter-specific hybrids into commercial cassava. Euphytica, 183(1), 19-26.
Anderson, P. (2005). Introduction. In “Whiteflies and Whitefly-Borne Viruses in the Tropics: Building a Knowledge Base for Global Action” (P. K. Anderson and F. Morales eds.), pp. 1–11. Centro Internacional de Agricultura Tropical, Cali, Colombia.
Andri, K. B., & Shiratake, Y. (2003). Existence, types and opportunities of contract farming in East Java. Bulletin of Faculty of Agriculture, Saga University (88), 43–55.
Arias, B. V., Bellotti, A. C., & Vargas, H. L. B. (2004, March). Nataima-31, a cassava (Manihot esculenta) variety resistant to the whitefly, Aleurotrachelus socialis. In Sixth International Scientific Meeting of the Cassava Biotechnology Network (pp. 8-14).
Bellotti, A. C., Smith, L., & Lapointe, S. L. (1999). Recent advances in cassava pest management. Annual Review of Entomology, 44(1), 343-370.
Bellotti, A., Bohorquez, A., Vargas, J., Arias, B., Vargas, H.L., Trujillo, G., MBA, C., Duque, M.C., Tohme, J. (2003). Recent advances in host plant resistance to whiteflies. Poster. CIAT. AA 6713. Accessed 5.19.13: http://www.tropicalwhiteflyipmproject.cgiar.org/docs/posters/PosterWhiteflyEnglish2003Socolen.pdf
Bellotti, A., Campo, B. V. H., & Hyman, G. (2012a). Cassava production and pest management: present and potential threats in a changing environment. Tropical Plant Biology, 5(1), 39-72.
Bellotti, A.C., Herrera, C.J., Hernadez, M.P., Arias, B., Guerrero, J.M., & Melo, E. (2012b). Chapter 10: cassava Pests in Latin America, Africa, and Asia. In Howeler, R. H. (Ed.) The cassava handbook: A reference manual based on the Asian regional cassava training course, held in Thailand. CIAT & The Nippon Foundation. Accessed 5.19.13: http://ciat-library.ciat.cgiar.org/Articulos_ciat/biblioteca/The%20Cassava%20Handbook%202011.pdf#page=266
Bohorquez, A., Lopez-Lavalle, B., Bellotti, A., & Tohme, J. (2011). Unraveling whitefly resistance in cassava. Poster. CIAT Agrobiodiversity and Biotechnology Project, AA6713. Accessed 5.19.13: http://ciat-library.ciat.cgiar.org/Articulos_Ciat/biblioteca/Unraveling%20ehitefly%20resistance%20in%20cassava.pdf
Bull, S. E., Ndunguru, J., Gruissem, W., Beeching, J. R., & Vanderschuren, H. (2011). Cassava: constraints to production and the transfer of biotechnology to African laboratories. Plant cell reports, 30(5), 779-787.
Carabalí, A., Belloti, A. C., & Montoya-Lerma, J. (2010). Biological parameters of Bemisia tabaci (Gennadius) biotype B (Hemiptera: Aleyrodidae) on Jatropha gossypiifolia, commercial (Manihot esculenta) and wild cassava (Manihot flabellifolia and M. carthaginensis) (Euphorbiaceae). Neotropical entomology, 39(4), 562-567.
Carabalí, A., Bellotti, A. C., Montoya-Lerma, J., & Cuellar, M. E. (2005). Adaptation of Bemisia tabaci biotype B (Gennadius) to cassava, Manihot esculenta (Crantz). Crop Protection, 24(7), 643-649.
Casida, J. E., & Quistad, G. B. (1998). Golden age of insecticide research: past, present, or future?. Annual review of
EVANS SCHOOL POLICY ANALYSI S AND RESEARCH (EPAR) |
11
Castle, S. J., Palumbo, J. C., Prabhaker, N., Horowitz, A. R., & Denholm, I. (2010). Ecological determinants of Bemisia tabaci resistance to insecticides. In Bemisia: Bionomics and management of a global pest (pp. 423-465). Springer, Netherlands.
Catholic Relief Services (n.d.) Final report on the great lakes cassava initiative. Accessed online: http://www.crsprogramquality.org/storage/pubs/agenv/glci-final-report.pdf
Colvin J, Omongo C.A., Maruthi M.N., Otim-Nape G.W., Thresh J.M. (2004). Dual begomovirus infections and high Bemisia tabaci populations: two factors driving the spread of a cassava mosaic disease pandemic. Plant Patholology, 53, 577-84; DOI: 10.1111/j.0032-0862.2004.01062.x.
De Barro, P. J., & Coombs, M. T. (2009). Post-release evaluation of Eretmocerus hayati Zolnerowich and Rose in
Australia. Bulletin of entomological research, 99(2), 193.
Ewusie, E. A., Parajulee, M. N., Adabie-Gomez, D. A., & Wester, D. (2010). Strip cropping: a potential IPM tool for reducing whitefly, Bemisia tabaci Gennadius (Homoptera: Aleyrodidae) infestations in cassava. West African Journal of Applied Ecology, 17(1).
FAO (2010). Cassava diseases in Africa: A major threat to food security. Strategic Programme Framework, 2010-2015.
Fargette, D., & Fauquet, C. (1988). A preliminary study on the influence of intercropping maize and cassava on the spread of African cassava mosaic virus by whiteflies. Aspects of Applied Biology, 17, 195-202.
Fargette, D., Fauquet, C., Grenier, E., & Thresh, J. M. (1990). The spread of African cassava mosaic virus into and within cassava fields. Journal of Phytopathology, 130(4), 289-302.
Fargette, D., Konate, G., Fauquet, C., Muller, E., Peterschmitt, M., & Thresh, J. M. (2006). Molecular ecology and emergence of tropical plant viruses. Annual Review of Phytopathology, 44, 235-260.
Faria, M., & Wraight, S. P. (2001). Biological control of Bemisia tabaci with fungi. Crop protection, 20(9), 767-778.
Fondong, V. N., Thresh, J. M., & Zok, S. (2002). Spatial and temporal spread of cassava mosaic virus disease in cassava grown alone and when intercropped with maize and/or cowpea. Journal of Phytopathology, 150(7), 365-374.
Gao, S., Qu, J., Chua, N. H., & Ye, J. (2010). A new strain of Indian cassava mosaic virus causes a mosaic disease in the biodiesel crop Jatropha curcas. Archives of virology, 155(4), 607-612.
Global Invasive Species Database. (2005). Website. Accessed 5.19.13: http://www.issg.org/database/welcome/
Gold, C. S., Altieri, M. A., & Bellotti, A. C. (1990). Direct and residual effects of short duration intercrops on the cassava whiteflies Aleurotrachelus socialis and Trialeurodes variabilis (Homoptera: Aleyrodidae) in Colombia. Agriculture, Ecosystems & Environment, 32(1), 57-67.
Gould, J., Waldner, D., Colletto, N., & Merten, P. (2008). Release and recovery of four species of Eretmocerus against Bemisia tabaci biotype B in Arizona. In Classical Biological Control of Bemisia tabaci in the United States-A Review of Interagency Research and Implementation (pp. 191-204). Springer Netherlands.
Hilje, L., Costa, H. S., & Stansly, P. A. (2001). Cultural practices for managing Bemisia tabaci and associated viral diseases. Crop Protection, 20(9), 801-812.
Horowitz, A. R., Antignus, Y., & Gerling, D. (2011). Management of Bemisia tabaci whiteflies. In The Whitefly, Bemisia tabaci (Homoptera: Aleyrodidae) Interaction with Geminivirus-Infected Host Plants (pp. 293-322). Springer Netherlands.
Islami, T., Guritno, B., & Utomo, W. H. (2011). Performance of cassava (Manihot esculenta Crantz) based cropping systems and associated soil quality changes in the degraded tropical uplands of East Java, Indonesia. Journal of Tropical Agriculture, 49(1-2), 31-39.
EVANS SCHOOL POLICY ANALYSI S AND RESEARCH (EPAR) |
12
Legg, J. P. (1994). Bemisia tabaci: The whitefly vector of cassava mosaic geminiviruses in Africa: An ecological perspective. African Crop Science Journal, 2(4), 437-448.
Legg, J. P. (1999). Emergence, spread and strategies for controlling the pandemic of cassava mosaic virus disease in east and central Africa. Crop Protection, 18(10), 627-637.
Legg, J. P., & Fauquet, C. M. (2004). Cassava mosaic geminiviruses in Africa. Plant molecular biology, 56(4), 585-599.
Legg, J. P., & Thresh, J. M. (2000). Cassava mosaic virus disease in East Africa: a dynamic disease in a changing environment. Virus Research, 71(1), 135-149.
Legg, J. P., Owor, B., Sseruwagi, P., & Ndunguru, J. (2006). Cassava mosaic virus disease in East and Central Africa: epidemiology and management of a regional pandemic. Advances in virus research, 67, 355-418.
Legg, J., Gerling, D., & Neuenschwander, P. (2003). Biological control of whiteflies in Sub-Saharan Africa. Biological control in IPM systems in Africa. CAB International, Wallingford, 87-100.
Legg, J.P. (2009) The cassava scourge. R4D Review. IITA. Issue 3, September 2009.
Liu, S. S., De Barro, P. J., Xu, J., Luan, J. B., Zang, L. S., Ruan, Y. M., & Wan, F. H. (2007). Asymmetric mating interactions drive widespread invasion and displacement in a whitefly. Science, 318(5857), 1769-1772.
Maruthi MN, Hillocks RJ, Mtunda K, Raya MD, Muhanna M, Kiozia H, et al. Transmission of Cassava brown streak virus by Bemisia tabaci (Gennadius). J Phytopathol 2005; 153:307-12; DOI: 10.1111/j.1439-0434.2005.00974.x.
Mbanzibwa, D. R., Tian, Y. P., Tugume, A. K., Patil, B. L., Yadav, J. S., Bagewadi, B., ... & Valkonen, J. P. T. (2011). Evolution of cassava brown streak disease-associated viruses. Journal of General Virology, 92(4), 974-987.
Morales, F. J. (2006). Tropical whitefly IPM project. Advances in virus research, 69, 249-311.
Moreau, T. L., & Isman, M. B. (2011). Trapping whiteflies? A comparison of greenhouse whitefly (Trialeurodes vaporariorum) responses to trap crops and yellow sticky traps. Pest management science, 67(4), 408-413.
Mware, B., Narla, R., Amata, R., Olubayo, F., Songa, J., Kyamanyua, S., & Ateka, E. M. (2009). Efficiency of cassava brown streak virus transmission by two whitefly species in coastal Kenya. Journal of General and Molecular Virology, 1(4), 040-045.
Navas-Castillo, J., Fiallo-Olivé, E., & Sánchez-Campos, S. (2011). Emerging virus diseases transmitted by whiteflies. Annual review of phytopathology, 49, 219-248.
Night, G., Asiimwe, P., Gashaka, G., Nkezabahizi, D., Legg, J. P., Okao-Okuja, G., ... & Mutumwinka, M. (2011). Occurrence and distribution of cassava pests and diseases in Rwanda. Agriculture, Ecosystems & Environment, 140(3), 492-497.
Njoku, D. N., & Muoneke, C. O. (2008). Effect of cowpea planting density on growth, yield and productivity of component crops in cowpea/cassava intercropping system. Agrosci. J, 7(2), 106-113.
Okogbenin, E., Egesi, C. N., Olasanmi, B., Ogundapo, O., Kahya, S., Hurtado, P., ... & Fregene, M. (2012). Molecular Marker Analysis and Validation of Resistance to Cassava Mosaic Disease in Elite Cassava Genotypes in Nigeria. Crop Science, 52(6), 2576-2586.
Olasantan, F. O., Ezumah, H. C., & Lucas, E. O. (1996). Effects of intercropping with maize on the micro-environment, growth and yield of cassava. Agriculture, ecosystems & environment, 57(2), 149-158.
Oliveira, M. R. V., Henneberry, T. J., & Anderson, P. (2001). History, current status, and collaborative research projects for Bemisia tabaci. Crop Protection, 20(9), 709-723.
EVANS SCHOOL POLICY ANALYSI S AND RESEARCH (EPAR) |
13
Omongo, C. A., Kawuki, R., Bellotti, A. C., Alicai, T., Baguma, Y., Maruthi, M. N., ... & Colvin, J. (2012). African Cassava Whitefly, Bemisia tabaci, Resistance in African and South American Cassava Genotypes. Journal of Integrative Agriculture, 11(2), 327-336.
Otim, M., Kyalo, G., Kyamanywa, S., Asiimwe, P., Legg, J. P., Guershon, M., & Gerling, D. (2008). Parasitism of Bemisia tabaci (Homoptera: Aleyrodidae) by Eretmocerus mundus (Hymenoptera: Aphelinidae) on cassava. International Journal of Tropical Insect Science, 28(3), 158.
Oyetunde, A. (n.d.) Canopy Development. IITA. Accessed 5.19.13: http://old.iita.org/cms/details/trn_mat/irg55/irg55cre.html.
Patil, .B L., & Fauquet, C. M. (2009). Cassava mosaic geminiviruses: actual knowledge and perspectives. Molecular plant
pathology, 10(5), 685-701.
Patil, B., L., Ogwok, E., Wagaba, H., Mohammed, I. U., Yadav, J. S., Bagewadi, B., ... & Fauquet, C. M. (2011). RNAi-mediated resistance to diverse isolates belonging to two virus species involved in Cassava brown streak disease. Molecular plant pathology, 12(1), 31-41.
Prochnik, S., Marri, P. R., Desany, B., Rabinowicz, P. D., Kodira, C., Mohiuddin, M., ... & Rounsley, S. (2012). The cassava genome: current progress, future directions. Tropical plant biology, 5(1), 88-94.
Pypers, P., Sanginga, J. M., Kasereka, B., Walangululu, M., & Vanlauwe, B. (2011). Increased productivity through integrated soil fertility management in cassava–legume intercropping systems in the highlands of Sud-Kivu, DR Congo. Field Crops Research, 120(1), 76-85.
Rusike, J., Mahungu, N. M., Jumbo, S., Sandifolo, V. S., & Malindi, G. (2010). Estimating impact of cassava research for development approach on productivity, uptake and food security in Malawi. Food policy, 35(2), 98-111.
Sseruwagi, P., Maruthi, M. N., Colvin, J., Rey, M. E. C., Brown, J. K., & Legg, J. P. (2006). Colonization of non-cassava plant species by cassava whiteflies (Bemisia tabaci) in Uganda. Entomologia experimentalis et applicata, 119(2), 145-153.
Sseruwagi, P., Sserubombwe, W. S., Legg, J. P., Ndunguru, J., & Thresh, J. M. (2004). Methods of surveying the incidence and severity of cassava mosaic disease and whitefly vector populations on cassava in Africa: a review. Virus research, 100(1), 129-142.
Stansly P.A., McKenzie C.L. (2008). Fourth International Bemisia Workshop International Whitefly Genomics Workshop. 53pp. Journal of Insect Science 8:4, available online: insectscience.org/8.04
Taylor, N. J., Halsey, M., Gaitán-Solís, E., Anderson, P., Gichuki, S., Miano, D., Bua, A., Alicai, T., & Fauquet, C. M. (2012). The VIRCA project: virus resistant cassava for Africa. GM Crops and Food: Biotechnology in Agriculture and the Food Chain, 3(2), 93-103.
Thresh, J. M., & Cooter, R. J. (2005). Strategies for controlling cassava mosaic virus disease in Africa. Plant pathology, 54(5), 587-614.
Vanderschuren, H., Moreno, I., Anjanappa, R. B., Zainuddin, I. M., & Gruissem, W. (2012). Exploiting the combination of natural and genetically engineered resistance to cassava mosaic and cassava brown streak viruses impacting cassava production in Africa. PloS one, 7(9), e45277.
Venkatesan, S., & Palanisamy, V. (2010). Eco-friendly management of cassava whitefly, Bemisia tabaci gennadius. Madras Agricultural Journal, 97(1/3), 78-80
Yaninek, J. S., James, B. D., & Bieler, P. (1994). Ecologically sustainable cassava plant protection (ESCaPP): a model for environmentally sound pest management in Africa. African Crop Science Journal, 2(4), 553-562.
Family Betaflexiviridae Cowpea mild mottle virus CPMMV Genus Carlavirus Melon yellowing-associated virus MYaV Family Potyviridae* Cassava brown streak virus CBSV Genus Ipomovirus Cucumber vein yellowing virus CVYV
Squash vein yellowing virus SqVYV
Sweet potato mild mottle virus SPMMV
Family Secoviridae Tomato torrado virus ToTV Genus Torradovirus Tomato marchitez virus ToMarV Proposed species: Tomato chocolàte virus ToChV
Tomato chocolate spot virus ToChSV
*Ugandan cassava brown streak virus (UCBSV) was also identified (Mbanzibwa et al., 2011)
EVANS SCHOOL POLICY ANALYSI S AND RESEARCH (EPAR) |
21
Appendix 2: Natural enemies of whitefly on cassava (from Bellottii et al., 2012b)
Principal Species Parasitoids Predators Entomopathogens (fungus) Aleurotrachelus sociales Amitus macgowni Delphastus sp Beauveria bassiana
E. americana D. quinculus lecanicillium lecani E. bellotti D. pusillus Aschersonia aleyrodes E. cubensis Chrysopa sp. nr. cincta Encarsia hispida Condylostylus sp. E. luteola E. sophia Encarsia sp. nr. variegata Encarsia sp. E. tabacivora Euderomphale sp. Eretmocerus spp. Metaphycus sp. Signiphora aleyrodis
Aleurothrixus aepim Encarsia porteri Cladosporium sp. E. aleurothrixi E. hispida Eretmocerus sp.
Bemisia tuberculata E. hispida Condylostylus sp. E. pergandiella E. sophia Encarsia sp.prob. variegata E. tabacivora Eretmocerus sp. Euderomphale sp. Metaphycus sp.
Bemisia tabaci Encarsia sophia Delphastus pusillus E. lutea Condylostylus sp. E. Formosa E. mineoi Encarsia sp. Eretmocerus mundus
Trialeurodes variabilis E. bellotti Chrysopa sp. nr. cincta Aschersonia aleyrodes E. hispida Condylostylus sp. Beauveria bassiana E. luteola Lecanicillium lecani E. nigricephala E. pergandiella Encarsia sp. E. sophia E. strenua E. tabacivora Eretmocerus spp.
Trialeurodes vaporariorum Encarsia tabacivora
EVANS SCHOOL POLICY ANALYSI S AND RESEARCH (EPAR) |
22
Appendix 3: Notable Researchers
• James Legg ([email protected]) o IITA plant virologist, Tanzania, 1386 citations, Great Lakes Cassava Initiative (BMGF funded) o Biography: http://www.iita.org/legg-james
• Anthony Bellotti o CIAT Cassava Program entomologist, Colombia o Information: http://www.ciatnews.cgiar.org/2013/03/06/the-passing-of-tony-bellotti/
• Pamela Anderson o International Potato Center (CIP) Director General, Peru o Biography: http://cipotato.org/about-cip/board/pamela-anderson
EVANS SCHOOL POLICY ANALYSI S AND RESEARCH (EPAR) |
23
Appendix 4: Whitefly Control Studies by Control Mechanism
Citation Year Region(s) Main Findings Organization(s) Breeding for Host Plant Resistance
Akinbo, O., Labuschagne, M., & Fregene, M. (2012). Introgression of whitefly (Aleurotrachelus socialis) resistance gene from F1 inter-specific hybrids into commercial cassava. Euphytica, 183(1), 19-26.
2012 Colombia- with relevance for Latin America and Africa
Confirmed introgression of resistance to A. socialis after evaluating 227 cassava genotypes for leaf damage and found 17.8% of genotypes highly resistant and promising for future breeding.
CIAT, Colombia; Department of Plant Sciences, University of the Free State, South Africa; Donald Danforth Plant Science Center, St. Louis, USA.
Omongo, C. A., Kawuki, R., Bellotti, A. C., Alicai, T., Baguma, Y., Maruthi, M. N., ... & Colvin, J. (2012). African Cassava Whitefly, Bemisia tabaci, Resistance in African and South American Cassava Genotypes. Journal of Integrative Agriculture, 11(2), 327-336.
2012 Colombia- with relevance for Latin America and Africa
All improved, high yielding CMD resistant varieties were highly susceptible to B. tabaci; several Ugandan landraces and South American genotype MEcu 72 showed greatest resistance.
National Crops Resources Research Institute, Uganda; Natural Resources Institute, UK; CIAT, Colombia
Intercropping and Planting Techniques
Night, G., Asiimwe, P., Gashaka, G., Nkezabahizi, D., Legg, J. P., Okao-Okuja, G., ... & Mutumwinka, M. (2011). Occurrence and distribution of cassava pests and diseases in Rwanda. Agriculture, Ecosystems & Environment, 140(3), 492-497.
2011 Rwanda
In an observational survey, whitefly populations were higher on improved variety cassava, though CMD was lower; intercropping was associated with lower pest populations (whitefly and green mite) and lower disease incidence and severity.
Institut des Sciences Agronomiques du Rwanda, Rwanda; University of Arizona, USA; IITA, Uganda and Tanzania
Ewusie, E. A., Parajulee, M. N., Adabie-Gomez, D. A., & Wester, D. (2010). Strip cropping: a potential IPM tool for reducing whitefly, Bemisia tabaci Gennadius (Homoptera: Aleyrodidae) infestations in cassava. West African Journal of Applied Ecology, 17(1).
2010 Ghana
Significantly lower numbers of immature and adult B. tabaci were found on cassava plots surrounded by five rows of cotton and jatropha.
Biotechnology and Nuclear Agriculture Research Center, Ghana; Texas Tech University, USA
Fondong, V. N., Thresh, J. M., & Zok, S. (2002). Spatial and temporal spread of cassava mosaic virus disease in cassava grown alone and when intercropped with maize and/or cowpea. Journal of Phytopathology, 150(7), 365-374.
2002 Cameroon
Intercropping with maize or cowpea reduced whitefly population by 50% and incidence of CMD by 20% in unimproved or semi-improved varieties of cassava, but not in the improved variety on experimental plots.
Department of Plant Pathology, Cornell University, USA; Natural Resources Institute, University of Greenwich, UK; Edona Research Centre, Cameroon
Fargette, D., Fauquet, C., Grenier, E., & Thresh, J. M. (1990). The spread of African cassava mosaic virus into and within cassava fields. Journal of Phytopathology, 130(4), 289-302.
1990 Cote d'Ivoire Lower incidence of ACMD (directly related to whitefly populations) was found on high density plots.
Scottish Crop Research Institute, UK; Washington University, USA; Laboratoire de Biomere, Franc; Overseas Development Natural Resources Institute
Gold, C. S., Altieri, M. A., & Bellotti, A. C. (1990). Direct and residual effects of short duration intercrops on the cassava whiteflies Aleurotrachelus socialis and Trialeurodes variabilis (Homoptera: Aleyrodidae) in Colombia. Agriculture, Ecosystems & Environment, 32(1), 57-67.
1990 Colombia
Cassava intercropped with maize or with cowpeas had significantly lower densities of whitefly eggs on leaves than monocropped experimental plots.
Division of Biological Control, University of California Berkeley, USA; CIAT, Colombia
EVANS SCHOOL POLICY ANALYSI S AND RESEARCH (EPAR) |
24
Fargette, D., & Fauquet, C. (1988). A preliminary study on the influence of intercropping maize and cassava on the spread of African cassava mosaic virus by whiteflies. Aspects of Applied Biology, 17, 195-202.
1988 Cote d'Ivoire
On experimental plots whitefly populations were lower on cassava plants intercropped with maize at high density than low density, but not significantly different from monocropped cassava plots.
Laboratoire de Vriologie, Institut Francais de Recherche pour le Devloppment en Cooperation (ORSTROM) Cote d'Ivoire.
Citation Year Region(s) Main Findings Organization(s) Insecticide
Dennehy, T. J., & Williams, L. (1997). Management of resistance in Bemisia in Arizona cotton. Pesticide science, 51(3), 398-406.
1997 Arizona, USA
This study incorporated two new integrated resistance elements: once-per-year use of the insect growth regulators (IGRs) pyriproxyfen and buprofezin, and measures to delay use of pyrethroids for as long into the growing season as possible. Through regimented timing of application and sampling plans, results indicated improvement in B. tabaci control and reduced insecticide use.
University of Arizona, Department of Entomology; Arizona Cotton Growers Association; Cotton Incorporated; USDA-ARS Western Cotton Research Laboratory; University of Arizona College of Agriculture
Denholm, I., Cahill, M., Dennehy, T. J., & Horowitz, A. R. (1998). Challenges with managing insecticide resistance in agricultural pests, exemplified by the whitefly Bemisia tabaci. Philosophical Transactions of the Royal Society of London. Series B: Biological Sciences, 353(1376), 1757-1767.
1998 North America, Israel
Studies in Israel and the SW United States have succeeded in arresting the resistance treadmill in B. tabaci through a combination of increased chemical diversity, voluntary or mandatory restrictions on the use of key insecticides, and careful integration of chemical control with other pest–management options, increasing the prospect of sustained use of existing and future insecticides.
Dept. of Biological and Ecological Chemistry, IACR-Rothamsted; Dept. of Entomology, University of Arizona; Dept. of Entomology, Institute of Plant Protection, Volcani Center, Israel
Ellsworth, P. C., & Martinez-Carrillo, J. L. (2001). IPM for Bemisia tabaci: a case study from North America. Crop Protection, 20(9), 853-869.
2001 North America
Insecticide resistance management strategies based on the structured and restricted use of non-neurotoxic IGRs, coupled with the use of cultural and biological pest management tactics, presently provides the best model for combating insecticide resistance in B. tabaci.
Maricopa Agricultural Center, University of Arizona
Palumbo, J. C., Horowitz, A. R., & Prabhaker, N. (2001). Insecticidal control and resistance management for Bemisia tabaci. Crop Protection, 20(9), 739-765.
2001 North America, Israel
Producers in the U.S. have had the greatest success with novel insecticide chemistries such as Nicotinoids, Imadacloprid soil treatments, second-generation nicotinoids, and non-neurotoxic insect growth regulators (IGRs) such as buprofezin and pyriproxyfen (Palumbo, et al., 2001).
University of Arizona, Yuma
EVANS SCHOOL POLICY ANALYSI S AND RESEARCH (EPAR) |
25
Horowitz, A. R., Gorman, K., Ross, G., & Denholm, I. (2003). Inheritance of pyriproxyfen resistance in the whitefly, Bemisia tabaci (Q biotype). Archives of insect biochemistry and physiology, 54(4), 177-186.
2003 UK
Two parental strains of B. tabaci belonging to the Q biotype were assayed with pyriproxyfen, and the resistance ratio and statistical modeling indicated that reistance was incompletely or partially dominant. Resistants to pyriproxyfen is conferred primarily by a mutant allele at a single locus.
Organizations: Gilat Research Center, Israel; Rothamsted Research, UK
Carrière, Y., Ellers-Kirk, C., Hartfield, K., Larocque, G., Degain, B., Dutilleul, P., ... & Tabashnik, B. E. (2012). Large-scale, spatially-explicit test of the refuge strategy for delaying insecticide resistance. Proceedings of the National Academy of Sciences, 109(3), 775-780.
2012 Arizona, USA
Refuge strategy effective in preventing resistance to pyriproxyfen when nearby spring melon, alfalfa, and cotton not treated with insecticides provide refuge for B. tabaci and promote survival or susceptible pests. Cotton refuges delayed resistance while treated cotton fields accelerated it.
National Academy of Sciences, U of Arizona, McGill, Arizona Cotton Research and Protection Council
Citation Year Region(s) Main Findings Organization(s)
Biological Control (Parasitoids)
Venkatesan, S., & Palanisamy, V. (2010). Eco-friendly management of cassava whitefly, Bemisia tabaci gennadius. Madras Agricultural Journal, 97(1/3), 78-80.
2010 India
Application of sweetflag rhizome Acorus calamus 10D and neem seed kernal extract (NSKE) significantly reduced whitefly populations
Tapioca and Caster Research Station
Otim, M., Kyalo, G., Kyamanywa, S., Asiimwe, P., Legg, J. P., Guershon, M., & Gerling, D. (2008). Parasitism of Bemisia tabaci (Homoptera: Aleyrodidae) by Eretmocerus mundus (Hymenoptera: Aphelinidae) on cassava. International Journal of Tropical Insect Science, 28(3), 158.
2008 Uganda
Experiement comparing parisitoid activity of Eretmocerus mundus on glabrous leaf and hirsute leaf cassava varieties found that leaf hairiness was not a factor in parasitoid activity, though there were some behavior changes.
National Crops Resources Research Institute, Uganda; Crop Science Department, Uganda; University of Arizona, USA; IITA, Tanzania;National Resources Institute, UK; Tel Aviv University , Israel
Asiimwe, P., Ecaat, J. S., Otim, M., Gerling, D., Kyamanywa, S., & Legg, J. P. (2007). Life-table analysis of mortality factors affecting populations of Bemisia tabaci on cassava in Uganda. Entomologia experimentalis et applicata, 122(1), 37-44.
2006 Uganda
Observational study of sources and rates of B. tabaci mortality on cassava in Uganda (post CMD epidemic) found parasitism to be highest cause of mortality across all stages, followed by dislodgement, predation, inviable eggs, and unknown causes.
IITA, Uganda; University of Greenwich, UK
De Barro, P. J., & Coombs, M. T. (2009). Post-release evaluation of Eretmocerus hayati Zolnerowich and Rose in Australia. Bulletin of entomological research, 99(2), 193.
2009 Australia
Evaluation of the 2004 release of Eretmocerus hayati showed increased parasitism, with Er. hayati contributing 85% of parasitism
CSIRO Ecosystem Sciences
Biological Control (Fungal)
EVANS SCHOOL POLICY ANALYSI S AND RESEARCH (EPAR) |
26
Al-Deghairi, M. A. (2008). Bioassay evaluation of the entomopathogenic fungi, Beuveria bassiana Vuellemin against eggs and nymphs of Bemisia tabaci Gennadius (Homoptera: Aleyrodidae). Pakistan J. Biol. Sci, 11(12), 1551-1560.
2008 Saudi Arabia
Fungal control study of Beauveria bassiana (squash) on B. tabaci eggs, and young and old nymphs: found nymphs more susceptible than eggs.
Qassim university, Saudi Arabia
Wraight, S. P., Carruthers, R. I., Jaronski, S. T., Bradley, C. A., Garza, C. J., & Galaini-Wraight, S. (2000). Evaluation of the Entomopathogenic Fungi Beauveria bassiana and Paecilomyces fumosoroseus for Microbial Control of the Silverleaf Whitefly, Bemisia argentifolii. Biological Control, 17(3), 203-217.
2000 Texas, USA
Fungal control study of Beauveria bassiana and Paecilomyces fumososeus against Bemisi argentifolii found high efficacy against nymphs, but minimal effectiveness against adults. Suggests the pathogens have strong potential for controlling whiteflies in cucurbit crops (melons and cucumbers).
USDA, USA; Mycotech Corporation, USA
Appendix 5: Whitefly Control Intervention Programs (1990-present)
Project Organizations Years Region(s) Project Overview Whitefly Component
IITA's Plant Health Management Division (PHMD), UNDP-funded
1993-1997
Benin, Cameroon, Ghana, Nigeria
The Ecologically Sustainable Cassava Plant Protection (ESCaPP) project which began in 1993 in West Africa provides such a working model. Researchers and extension agents training farmers in principles and practices of sustainable crop production and protection.
Tested and adapted sustainable cassava plant protection technologies for the most important arthropod, pathogen and weed pests in West Africa.
Five subprojects: (1) Bemisia tabaci as a vector of viruses affecting cassava and sweet potato in sub-Saharan Africa (IITA, NRI, CIP, CIAT); (2) B. tabaci as a vector of viruses in mixed cropping systems of Mexico, Central America, and the Caribbean (CIAT); (3) B. tabaci as a vector of viruses in mixed cropping systems of eastern and southern Africa (ICIPE, AVRDC); (4) B. tabaci as a vector of viruses in mixed cropping systems of Southeast Asia (AVRDC); (5) Trialeurodes vaporariorum as a pest in mixed cropping systems of the Andean highlands (CIAT); and (6) whiteflies as pests of cassava in South America (CIAT).
Provided crisis mitigation for CMD pandemic in Tanzania and Uganada; validated IPM components; and developing training materials characterized and targeted “hot spots” surrounding Lake Victoria in Uganda and Tanzania. Developed integrated pest management (IPM) components and packages that are safe and affordable for small-scale farmers. Also strengthened the tropical whitefly research network and provided advanced training (technical, M.Sc., Ph.D.) and IPM information for plant protection specialists from other institutions. Integration of different IARCs and IRS scientists. Project showed that “genetic improvement is the most sustainable component of an IPM Programme.”
EVANS SCHOOL POLICY ANALYSI S AND RESEARCH (EPAR) |
27
Sustainable Integrated Management of Whiteflies as Pests and Vectors of Plant Viruses in the Tropics
CGIAR Systemwide Programme on IPM. CIAT-led, in partnership with IARCs, Advanced Research Intstitute and local NARS.
2001-2004
Colombia, Ecuador, El Salvador, Guatemala, Uganda, United Republic of Tanzania
Promote IPM packages and training materials to address whiteflies as pests in tropical highlands, vectors in mixed cropping systems, and as virus vectors and pests in cassava.
This subproject of the Tropical Whitefly IPM Project intends to scale out by gathering, generating, and analyzing baseline data relevant to the diagnosis and characterization of whitefly and WTV problems in the tropics in order to propose a sound research agenda for improved understanding of pest and disease dynamics, IPM development and IPM implementation. Results of various projects summarized in Anderson (2005).
Integrated Protection of Cassava from Emerging Pests and Diseases that Threaten Rural Livelihoods
IITA 2007-2010
Benin, Cameroon, DR, Congo, Guinea, Tanzania
Aimed to test and implement sustainable IPM technologies to mitigate losses to major cassava pests and diseases in five countries, including DRC and Tanzania, including evaluating pest and disease resistant varieties and their multiplication and dissemination to farmers. It looked at the use of natural enemies to control pests and diseases, the spread of diseases and their insect vectors, and attempted to put together pest /disease management options and test them at farmer level.
Introduced into eastern Africa (first to Tanzania) parasitoids that were shown to be effective in controlling spiralling whitefly in West Africa. Efforts to bring parasitoids of spiralling whitefly succeed in establishing in East Africa aimed to bring pest under control at least in the coastal areas of Tanzania.
Project Organizations Years Region Project Overview Whitefly Component
Great Lakes Cassava Initiative (GLCI)
Catholic Relief Services 2007-2012
SSA:Burundi, Democratic Republic of Congo, Kenya, Rwanda, Tanzania and Uganda
Overall goal of distributing clean planting material of disease tolerant or resistant varieties to 1.15 million farmers to six countries. Multi-phase project also included research, farmer training, and capacity building in CBSVs diagnostics and response amongst GLCI countries. Follow-up project to C3P, focusing on both research and development activities.
Research to increase knowledge in CBSV transmission by whitefly. Results of CBSD field trials in Tanzania and Uganda suggests treating whiteflies with insecticides is effective in reducing whitefly populations and CMD incidence, but does not stop whitefly adults from entering treated plots and feeding on plants in those treated plots, which suggests contrasting transmission characters by whiteflies. This study also showed that the improved variety was much more resistant to CMD infection (near immune) than the local variety.
Enabling Research Tools for Cassava Virologists and Breeders
University of Greenwich Natural Resources Institute, partnered with Mikocheni Agricultural Research Institute, Tanzania (BMGF Funded)
2013-ongoing East Africa Training for East African researchers in
molecular biology techniques.
Expected to deliver diagnostic markers identifying gene sequences in superabundant whitefly populations.
Integrated Cassava Disease Interventions with Little or No Whitefly-Specific Component
EVANS SCHOOL POLICY ANALYSI S AND RESEARCH (EPAR) |
28
Emergency Programme to Combat the Cassava Mosaic Disease Pandemic in East Africa
USAID, IITA 1998-2001 East Africa
Emergency program to multiply and disseminate mosaic resistant cassava in Uganda, Kenya and Tanzania through clean cutting multiplication sites, germplasm diversification, and farmer training in partnership with local organizations.
No specific whitefly component, but CMD incidence data from leaf samples included both cutting- and whitefly-borne infection data.
C3P ON-FARM VOUCHERS: CRS 2006-2008 Eastern
Africa
Pilot Use of On-Farm Vouchers to Disseminate Cassava Planting Material in Western Kenya. Multiplication and dissemination of clean planting materials through on-farm vouchers in all six participating countries and was notable for developing Quality Management Protocol.
Cassava Mega Project
ASARECA, with funding from USAID
2008-2012 Eastern and Central Africa
Project to improve cassava productivity and utilization throughout the region with interventions at research, development, and policy levels. Activities include putting in place systems for generating quality cassava planting materials of improved cassava varieties and scaling up technologies for cassava production, processing and marketing. Policy activities aimed to put in place supportive policies to promote cassava production and processing and to develop appropriate quality standards.
Regional Cassava Initiative (RCI)
FAO, Humanitarian Aid dept of the European Commission (ECHO)
2006-present
Central and Eastern Africa
Emergency project funded by ECHO to address cassava diseases in Eastern and Central Africa through increasing access to improved cassava varieties, strengthening surveillance information to government authorities, NGOs, and donors, and promoting operational cassava commissions in each country to better regulate movement of cassava vegetative material throughout countries. Works in collaboration with GLCI and Cassava Mega Project.
![Identification whitefly[Referencia_Introducción]](https://static.documents.pub/doc/80x56/55cf99dc550346d0339f8ca9/identification-whiteflyreferenciaintroduccion.jpg)
